Earthquakes and Tsunamis

Earthquakes

• Late Thursday night (our time), an earthquake occurred of the coast of Japan.

Earthquakes

• Late Thursday night (our time), an earthquake occurred of the coast of Japan.

Earthquakes

• One of the largest earthquakes since we’ve been measuring them (6th strongest).

What is an Earthquake?

• Earthquakes are movements of the earth that release pressure.

• Occur along faults.

- planar breaks in rock where there is movement.

- occur in all sizes.

Faults

Faults

What Causes an Earthquake?

• Build up of strain (gradual or sudden) deforms the earth until the elastic limit is reached – then things snap.

• Elastic Rebound.

• Energy is released in the form of seismic waves.

What Causes an Earthquake?

How do we Measure an Earthquake?

• Most common way is by the Richter Scale.

• Based on the amplitude of the seismic waves.

• Scale is logarithmic! Difference in two whole numbers means in increase in vibrations by a factor of 10!

- ground that moves 1 inch in a 4.0 earthquake moves 10 inches in a 5.0 quake, and 100 inches in a 6.0 quake.

Where Does the Strain Come From?

• Plate tectonics.

• Outermost portion of our planet is broken into different pieces called plates.

• The plates move all the time.

Tectonic Plates

Plate Boundaries

• Most of the action occurs at the boundary between two plates.

• Plates can move away from one another, towards one another, or just slide past one another.

Plate Boundaries

Convergent Plate Boundaries

• When plates run into one another, the more dense plate is forced underneath the less dense plate, a process called subduction.

Convergent Plate Boundaries• When ocean material runs into continental material one gets a trench in the ocean, and a chain of volcanic mountains on the continent.

Convergent Plate Boundaries• When ocean material runs into continental material one gets a trench in the ocean, and a chain of volcanic mountains on the continent.

Convergent Plate Boundaries• When ocean material runs into ocean material one gets a trench in the ocean, and a chain of volcanic islands called an island-arc.

Convergent Plate Boundaries• When ocean material runs into ocean material one gets a trench in the ocean, and a chain of volcanic islands called an island-arc.

Earthquake Damage

• Ground movement

• Fire

• Landslides

• Liquefaction

• Tsunamis

Earthquakes and Tsunamis

• Sudden movement of the sea floor will displace a large volume of water.

Tsunami Speed

• The speed of the tsunami is controlled by the depth of the water.

C (celerity) = √ g x d

where g is gravitational acceleration, and d is the depth of the water in which the wave is travelling.

• Since g is a constant, the speed is mostly a function of water depth (wavelength also plays a role, but we’re neglecting it for now).

Tsunami Speed

• Average depth of the ocean is 4000 m.

C (celerity) = √ g x d

C = √ 9.8 m/s2 x 4000 m

C = √ 39,200 m2/s2

C = 198 m/s

• 1609 meters in a mile, 3600 seconds in an hour.

C = 198 m/s / 1609 m/mile x 3600 s/hr

C = 443 miles/hour!

Tsunami Speed

• As the wave approaches the shore its speed decreases.

C (celerity) = √ g x d

• Aside from slowing, other things happen to the wave as it enters shallower water.

Tsunami Occurrences

Indonesia, December 2004

• 9.1 magnitude earthquake occurred only 8 miles deep.

• Rupture was 1200 km long with 15 m of vertical displacement.

Indonesia, December 2004

Indonesia, December 2004

• Magnitude 8.9.

• Strong enough quake to affect our axis of rotation (a day on earth is now a little shorter).

Japan, March 2011